1. Field of the Invention
This invention relates to a height estimating apparatus and method suitable particularly, but not exclusively, for estimating a first height of a vehicle above a first reference surface.
2. Discussion of Prior Art
Conventional apparatus used in aircraft navigation systems to estimate height of the aircraft above a fixed reference surface typically include instruments such as the baro-altimeter to estimate height above sea level, radar altimeter to estimate height above ground level, and can also include various configurations of laser and radar devices to estimate a height from obstacles to the aircraft. The baro-altimeter measurement is combined with various outputs from the aircraft""s navigation system to provide a baro-inertial height, which is an estimate of the aircraft""s height above sea level. The baro-inertial height is the least accurate of the various height measurements described above because the performance of the baro-altimeter is dependent on atmospheric conditions and the flight dynamics of the aircraft.
Various systems are available as an alternative for estimating height above sea level, which are partially or wholly independent of the baro-altimeter input. These include using secondary surveillance radar, where a body external to the aircraft reports a measured height of the aircraft by radio, but this suffers from the limitation that someone or something is required to communicate the height to the aircraft and that the relevant surveillance instrumentation, additional to the navigation instrumentation on the aircraft, is required in order to make the measurements. Further alternatives include using the height output from a satellite range triangulation system such as the Global Positioning System (GPS) described in the Nato unclassified report STANAG 4294 (published by the military agency for standardisation) but this relies on satellite signals being available, or using the radar altimeter measurement, but as this measures height above ground level, it is unsuitable for mountainous terrain where the ground level is a significant distance from that of the sea.
There is thus a need for an improved height estimating apparatus which is substantially independent of flight dynamics and atmospheric conditions, and which can estimate height above sea level.
According to a first aspect of the present invention there is provided apparatus for estimating a first height of a vehicle above a first reference surface, including a system for determining position, velocity and attitude incorporating at least one sensing means operable to provide an output signal indicative of a vertical specific force of the vehicle, error-estimating means for receiving as input signals a horizontal reference velocity and position of the vehicle, and a radar altimeter measurement of a second height of the vehicle above a second reference surface and for providing as an output signal estimates of errors associated with the sensing means output signal, and integrating means for receiving said sensing means and error-estimating means output signals, and for subtracting the estimates of errors from the signal indicative of vertical specific force while performing a double integration of the results of the subtraction, to provide an output indicative of the required estimated first height.
Preferably the at least one sensing means is an inertial vertical specific force sensor operable to provide the output signal indicative of the vertical specific force of the vehicle.
Conveniently said first reference surface is sea level and said second reference surface is ground level.
Advantageously the error-estimating means includes a Kalman filter and a gravity corrector.
Preferably there are provided first, second and third estimator stations at which the output signal estimates of errors associated with the sensing means output signal are stored.
Conveniently the integrating means includes first and second subtractors and first and second integrators.
Advantageously the horizontal velocity and position of the vehicle forming an input to the Kalman filter is provided by the system for determining position, velocity and attitude.
According to a further aspect of the present invention there is provided a method for estimating a first height of a vehicle above a first reference surface, including the steps of operating at least one sensing means, forming part of a system for determining position, velocity and attitude of a vehicle, to provide an output signal indicative of a vertical specific force of the vehicle, inputting a horizontal velocity and position of the vehicle and a radar altimeter measurement of a second height of the vehicle above a second reference surface to error-estimating means, establishing in said error-estimating means estimates of errors associated with the sensing means output signal, and subtracting the estimates of errors from the sensing means output signal while performing a double integration of the results of the subtraction, to provide the required estimated first height.
Preferably said double integration includes a first integration, which first integration integrates the vertical specific force to provide a vertical velocity, and a second integration, which second integration integrates the vertical velocity in order to provide the required estimated first height.
Conveniently the at least one sensing means is a vertical specific force sensor, with estimates of a bias associated with the vertical specific force sensor being provided by a Kalman filter, together with estimates of a vertical velocity error and estimates of a vertical height error associated with the system for determining position, velocity and attitude.
Advantageously said subtraction of estimates of errors includes a first subtraction, which first subtraction is effected while performing the first integration and subtracts the estimate of the bias associated with the specific force sensor and a correction for gravity supplied by the gravity corrector from said vertical specific force on a continuous basis, together with a subtraction of the vertical velocity error at discrete intervals, and a second subtraction, which second subtraction is effected on the second integration at discrete intervals and subtracts the vertical height error estimate therefrom to provide the required estimated first height.
Preferably the double integration is effected at a processing cycle frequency of substantially 50 Hz and outputs the required estimated first height at an output rate of substantially 50 Hz, the radar altimeter measurement of a second height of the vehicle above a second reference surface is input to the estimating means at an input rate of substantially 12.5 Hz and the estimate of the bias of the vertical specific force sensor, the estimate of gravity, the vertical velocity error estimate and the vertical height error estimate are used to correct the double integration with a correction cycle frequency in the range of from 2 to 4 Hz.